• electron correlation;
  • Mott;
  • polaron;
  • semiconductor


Low dimensional systems are characterized by at least one spatial dimension of only some atoms. Such size reduction has often important consequences for physical properties. Electronic correlation and electron–phonon coupling can originate Mott insulators or charge density waves (CDWs), both phenomena enhanced by dimensionality reduction. Interfaces offer a natural way of reducing the dimensionality. Among all the surfaces, semiconducting surfaces are particularly well adapted for electronic correlation studies. In them, correlation is enhanced because of the low dimension, the electronic localization in dangling bonds and the large inter-orbital distances in reconstructions. Despite these factors favoring correlation, eventually stronger than in bulk systems, the field is by far much less developed. We review here the discovery of correlated surfaces, while studying the Schottky barrier of alkalis on Si or GaAs, and coetaneous studies on SiC. We summarize then the studies on K/Si(111):B, whose equation image was considered a surface Mott insulator with an important electron-phonon coupling. The recent discovery of a equation image symmetry has been first interpreted as evidence of a bipolaronic insulator, but new findings have finally proven it to be a band insulator. We will then focus on the model system Sn/Ge(111). The last unclear issue about the (3 × 3) reconstruction at 150 K was related to the Sn 4d core level. High-resolution photoemission has clarified the core level deconvolution while refining the structural model. This metallic reconstruction is sensitive to electronic correlations which trigger a phase transition to a Mott phase below 30 K.